In a CRT-based projection television (PTV), three CRTs project colored (RGB) images onto a rear projection screen. Typically, the images are reflected by a mirror, which lengthens the optical path to produce a larger output image. Because the three CRTs are positioned at three different locations, the projected images can be distorted in various ways. If these distortions are not corrected or “converged,” then the projected images are blurred and have objectionable color fringe artifacts.
Typically, convergence errors are measured and corrected at the factory prior to distribution. However, due to physical, electrical, optical, and magnetic factors, it is often difficult to maintain convergence over time. Many prior art PTV's provide manual adjustment to fine tune the convergence, but users prefer an automatic method.
Therefore, manufacturers have developed automatic methods for performing the convergence correction. U.S. Pat. No. 6,404,462 “Digital convergence image,” issued to George on Jun. 11, 2002, describes convergence correction in a PTV where individual photo sensors are positioned adjacent to projection screen edges. CRTs generate a raster image for illuminating the projection screen and the photo sensors. Signals derived from the photo sensors are used for convergence alignment. In another similar method, optical fibers are placed next to the screen to send light to detectors to determine alignment errors.
The prior art techniques generally measure alignment at a few isolated locations. At best the locations are at an extreme periphery of the projected image because placing the detectors on the screen would interfere with the projected images.